Understanding the unexpected linearity of the trans-{Mn(NO)2}8 unit in a phthalocyanine complex: some thoughts on dinitrosylheme intermediates in biology

Simple MO arguments provide a qualitative explanation for the near-linear ON-Mn-NO arrangement observed for the trans-{Mn(NO)2}8 anion [Mn(Pc)(NO)2]−, which is unexpected for an Enemark-Feltham electron count n > 6. The metal center in this species may be described as low-spin d6(“t2g6”) and the two unpaired electrons occupy a pair of eu orbitals composed of NO(π*) components, giving rise to a triplet ground state. In a certain sense, these eu SOMOs may be likened to the SOMO (singly occupied molecular orbital) of the allyl radical. The electronic structure of this species is quite different from that of diamagnetic dinitrosylheme intermediates, which have been spectroscopically characterized in synthetic studies as well as proposed for soluble guanylate cyclase and cytochrome c′. Some speculative remarks are offered as to why this proposal is not an unreasonable one from an electronic-structural perspective.